In subterranean drilling operations the need frequently arises to determine the relative location of the wellbore being drilled (the drilling well) with respect to a pre-existing offset wellbore (a target well). This need may exist for the purpose of avoiding a collision or making an interception, or for the purpose of maintaining a specified separation distance between the wells (e.g., as in well twinning operations such as steam assisted gravity drainage operations). Magnetic ranging techniques are commonly employed to determine the relative location of the target well, for example, by making magnetic field measurements in the drilling well. The measured magnetic field may be induced in part by ferromagnetic material or an electromagnetic source (or sources) in the target well such that the measured magnetic field vector may enable the relative location of the target well to be computed.
Existing magnetic ranging techniques are similar to conventional static surveys in that they require drilling to be halted and the drill string to be held stationary in the drilling well while each magnetic survey is obtained. Magnetic ranging operations are therefore costly and time consuming. Moreover, magnetic ranging is similar to wellbore navigation in that the well path may be continuously adjusted in response to the ranging measurements. It may therefore be desirable to make ranging measurements as close to the bit as possible, in order to gain the earliest possible notification of required course adjustments. Owing to the rotation of the bit, measurements made close to the bit while drilling are made from a rotating platform (i.e., with rotating magnetic field sensors). There is a need in the art for magnetic ranging methods that employ magnetic field measurements made from a rotating platform (rotating sensors).